Poly(isobutylene-co-isoprene) or IIR, is a synthetic elastomer commonly known as butyl rubber (or Butyl polymer) which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (usually not more than 2.5 mol %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, usually not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Halogenation of this butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company © 1987), particularly pp. 297-300.
The development of halogenated butyl rubber (halobutyl) has greatly extended the usefulness of butyl by providing much higher curing rates and enabling co-vulcanization with general purpose rubbers such as natural rubber and styrene-butadiene rubber (SBR). Butyl rubber and halobutyl rubber are high value polymers, as their unique combination of properties (excellent impermeability, good flex, good weatherability, co-vulcanization with high unsaturation rubbers, in the case of halobutyl) make them preferred materials for various applications, such as their use in making tire inner tubes and tire inner liners.
The presence of allylic halide functionalities allows for nucleophilic alkylation reactions. It has been recently shown that treatment of brominated butyl rubber (BIIR) with nitrogen and/or phosphorus based nucleophiles, in the solid state, leads to the generation of IIR-based ionomers with interesting physical and chemical properties (see: Parent, J. S.; Liskova, A.; Whitney, R. A; Resendes, R. Journal of Polymer Science, Part A: Polymer Chemistry 43, 5671-5679, 2005; Parent, J. S.; Liskova, A.; Resendes, R. Polymer 45, 8091-8096, 2004; Parent, J. S.; Penciu, A.; Guillen-Castellanos, S. A.; Liskova, A.; Whitney, R. A. Macromolecules 37, 7477-7483, 2004). The ionomer functionality is generated from the reaction of a nitrogen or phosphorous based nucleophile and the allylic halide sites in the halogenated butyl rubber to produce an ammonium or phosphonium ionic group respectively.
Like other rubbers, for most applications, butyl rubber must be compounded and vulcanized (chemically cross-linked) to yield useful, durable end use products. Grades of butyl rubber have been developed to meet specific processing and property needs, and a range of molecular weights, unsaturation, and cure rates. Both the end use attributes and the processing equipment are important in determining the right grade of butyl rubber to use in a specific application. The selection and ratios of the proper fillers, processing aids, stabilizers, and curatives also play critical roles in both how the compound will process and how the end product will behave.
Peroxide curable butyl rubber compounds offer several advantages over conventional sulfur-curing systems. Typically, these compounds display extremely fast cure rates and the resulting cured articles tend to possess excellent heat resistance. In addition, peroxide-curable formulations are considered to be “clean” in that they do not contain any extractable inorganic impurities (e.g., sulfur). The clean rubber articles can therefore be used, for example, in condenser caps, biomedical devices, pharmaceutical devices (stoppers in medicine-containing vials, plungers in syringes) and possibly in seals for fuel cells.
One approach for obtaining a peroxide-curable butyl-based formulation lies in the use of conventional butyl rubber in conjunction with a vinyl aromatic compound like divinylbenzene (DVB) and an organic peroxide (see Japanese Publication No. 06-107738). In place of DVB, an electron-withdrawing group-containing polyfunctional monomer (ethylene dimethacrylate, trimethylolpropane triacrylate, N,N′-m-phenylene dimaleimide) can also be used (Japanese Publication No. 06-172647).
A commercially available terpolymer based on isobutylene (IB), isoprene (IP) and DVB, XL-10000, is curable with peroxides alone. However, this material does possess some significant disadvantages. For example, the presence of significant levels of free DVB can present safety concerns. In addition, since the DVB is incorporated during the polymerization process a significant amount of crosslinking occurs during manufacturing. The resulting high Mooney (60-75 MU, ML1+8@ 125° C.) and presence of gel particles make this material extremely difficult to process. For these reasons, it would be desirable to have an isobutylene based polymer which is peroxide curable, completely soluble (i.e., gel free) and contains no, or trace amounts of divinylbenzene in its composition.
U.S. Pat. No. 5,578,682 discloses a process for obtaining a polymer with a bimodal molecular weight distribution derived from a polymer that originally possessed a monomodal molecular weight distribution. The polymer, e.g., polyisobutylene, a butyl rubber or a copolymer of isobutylene and para-methylstyrene, was mixed with a polyunsaturated crosslinking agent (and, optionally, a free radical initiator) and subjected to high shearing mixing conditions in the presence of organic peroxide. This bimodalization was a consequence of the coupling of some of the free-radical degraded polymer chains at the unsaturation present in the crosslinking co-agent.
Sudo et. al. (U.S. Pat. No. 5,994,465) discloses a method for curing regular butyl, with isoprene contents ranging from 0.5 to 2.5 mol %, by treatment with a peroxide and a bismaleimide species. Canadian Patent No. 2,418,884 and Canadian Patent Application No. 2,458,741 describe the preparation of butyl-based, peroxide-curable compounds which have high multiolefin content. Specifically, CA 2,418,884 describes the continuous preparation of DR with isoprene levels ranging from 3 to 8 mol %. The elevated level of isoprene in the polymer backbone renders these compounds peroxide curable. Halogenation of this high muitiolefin butyl rubber consumes some of this unsaturation and produces a reactive allylic halide functionality within the elastomer. With these elevated levels of isoprene, it is possible to generate BIIR analogues which contain allylic bromide functionalities ranging from 3 to 8 mol %, often with residual double bonds in the polymer backbone. Nucleophilic substitution reactions as described above can be used to create ionomeric moieties from these allylic halide sites, with the residual unsaturation being sufficient to permit peroxide curing. Peroxide curable butyl rubber ionomer compositions with elevated levels of isoprene are described PCT Publication Nos. WO2007/022618 and WO2007/022819.
However, production of the starting butyl rubber with high isoprene content can be economically disadvantageous in a continuous production environment, due to the need to transition between grades. It would therefore be desirable to have a peroxide curable butyl rubber ionomer composition based on standard grades of butyl rubber with multiolefin content of 2.5 mol % or less. The physical and dynamic properties of the low isoprene content peroxide cured butyl compounds and halobutyl compounds generally are poor.
It would therefore be desirable to provide self curable or peroxide ionomer compositions which do not require elevated levels of multiolefins.